1. Field of the Invention
The present invention relates to a pressure-sensitive copying sheet and, more particularly, it relates to a pressuresensitive copying sheet having an improved color former layer.
2. Description of the Prior Art
Recording sheets of the type which undergo a change in color from a colorless state to a colored state by pressure have long been known as a pressure-sensitive copying paper (e.g., as disclosed in U.S. Pat. Nos. 2,711,375; 2,712,507; 2,730,456; 2,730,457; 3,418,250; 3,432,327; etc.).
With these pressure-sensitive papers, the higher the density of the recorded images obtained by applying pressure, the more preferable is the paper. However, in using these pressure-sensitive copying papers for out-put recording of a computer, colored images having a sufficiently high density cannot be obtained. Because, the impact pressure of a key for out-put recording is so low (less than about 200 kg/cm.sup.2 on the average) thus the density of the recorded images is reduced. This low pressure is particularly conspicuous in the case of making a number of copies.
In general, for increasing the color density, it is known (1) to increase the color former concentration in the capsules; (2) to make the particle size of the microcapsules large to increase rupture efficiency; (3) to increase the amount of microcapsules coated; (4) to enhance the color-developing ability of a color developer; and the like.
However, since almost all color formers possess low solubility, increasing the color former concentration fails to sufficiently attain the objects. When the microcapsule particle size is increased, coloration disadvantageously occurs upon winding the paper up in production and in the processing steps, on cutting the paper or upon the accidental application of pressure (formation of smudges), leading to another defect. Also, increasing the amount of microcapsules coated decreases the flexibility of the coated paper. For these reasons, it is industrially difficult to increase the color density using the above-described approaches.
In addition, it has heretofore been suggested, to use a solid granular substance and/or a binder to prevent the accidental destruction of microcapsules. However, these substances reduce the coating ability, deteriorate the surface property of the coated surface, reduce the efficiency of rupturing the microcapsules and copying ability, and the like.